1. Field of the Invention
The present invention relates to a process for providing denitrification of a previously nitrified wastewater, wherein a natural biomass is collected within the wastewater, the endogenous respiration of the biomass creating an oxygen demand sufficient to promote bacterial denitrification without the need for an outside carbon source such as methanol or the like.
2. Discussion of the Prior Art
Nitrogen in various chemical forms is found in sewage and industrial wastes that in sufficient concentration may be considered a pollutant. It can serve as a nutrient for algae which with excessive nitrogen may cause nuisance algal blooms. Reduction of the nitrogen in effluent from wastewater treatment plants can help control these problems.
Nitrogen in its oxidized states (e.g. as nitrates or nitrites) can seep into ground waters, causing problems in drinking water. Drinking water standards generally limit the concentration of nitrate to 5 to 10 mg/l, yet effluent from a modern treatment plant may have natural levels greater than 20 mg/l. Nitrogen in its reduced state, as ammonia, is toxic to fish, and severe limits are in effect on many streams to control the maximum concentration.
A conventional method of nitrogen removal is by biological means. With sufficient time, oxygen, and the proper mass of microorganisms, organic nitrogen is biologically converted to ammonia and then further oxidized to nitrate forms. This conversion occurs under aerobic (with oxygen) conditions, and is relatively easy to accomplish, resulting naturally under different known types of waste treatment processes. At this point the nitrogen has not been reduced in concentration, only converted to a different form.
A practical means to remove nitrate is to convert them to nitrogen gas. At this point N.sub.2 will evolve from the water and become atmospheric nitrogen. As atmospheric nitrogen, it is not a water pollutant. Nitrates are best converted to nitrogen gas by microbial action. Under anoxic conditions (without free dissolved oxygen), many common bacteria with a demand for oxygen are able to biochemically remove the oxygen from the nitrate ion, leaving nitrogen gas. This process is called biological denitrification.
For denitrification to occur, the nitrogen must first be converted to nitrates and then the bacteria must have a food source to create a demand for oxygen. This food source may be from outside, like a chemical addition of methanol, by the addition of sewage, or by the natural demand of the organisms (endogenous respiration). This natural demand must occur under conditions where free oxygen is absent.
Denitrification is carried out in tanks constructed to hold the microbial mass in suspension. When sewage is used for the food source it must be used at the start of the biological process. The aeration tank contents containing the nitrates are recirculated to a separate tank and mixed with the sewage without aeration. The resultant demand converts the nitrates to nitrogen gas. This process is limited to intermediate removal levels only, as there is a practical limit to how much wastewater can be recirculated.
Conventionally, in order to obtain near complete nitrogen removal it is necessary to treat the entire stream after all nitrification has occurred and without any contamination by the ammonia in the raw waste. This requires a separate tank downstream of all nitrification processes and upstream of any final clarifying processes. At this point sewage cannot be used as the food source, and the carbon source is limited to a clean outside food source, such as added methanol, or the oxygen demand caused by the bacteria themselves.
The latter option of allowing the bacteria to create the oxygen demand is slow, and is related to time and the concentration of organisms. If the organism concentration throughout the process is doubled, the reaction will occur nearly twice as fast. However, there is a practical limit to how high one can raise the concentration of the organisms within the process. If a system is operated at a high microorganism concentration, the mass may be sufficient to promote good endogenous nitrate reduction. However the mass may be too high to promote good sedimentation and excess solids will be in the effluent. These solids contain organic nitrogen which will have the same detrimental effect in the streams.
Also, if solids are maintained at a high level throughout the process, they become old and have very little endogenous demand, and even though a high concentration may be obtained, denitrification will be slow or incomplete. Therefore it becomes quite difficult to operate denitrification without methanol addition at the lower solids levels required to meet other treatment objectives.
After the waste exits the denitrification tank, it must be aerated again to add free oxygen back to the waste stream. The purpose of this is to add needed oxygen back to the liquid and to strip nitrogen gas from the system to promote better sedimentation within the final sedimentation tank. If methanol has been added, oxygen is required to metabolize the excess methanol so it does not discharge into the streams and lakes.
The liquid from the first aeration tank is made up of a suspension of bacteria. This concentration may typically be in the range of 2000 to 3000 mg/l. As this stream enters the anoxic tank, it contains a residual oxygen from the first tank that must be used up before any denitrification can occur. The rate at which this deoxygenation and denitrification can occur is strictly related to the number and activity of the organisms. If the organisms can be increased in numbers, complete denitrification can occur, and can occur in a smaller tank.